Chinese patent document No. CN02135756 disclosed a tilting-type water-hardening process for treating steel slag. This process uses a water-quench tank and a stanchion beside the tank. On one side of the stanchion is a granulator. When melt steel slag was poured into a slag jar on a truck and carried to the water-quench tank, the slag jar is tilted by operating the drop hanger of the windlass and the lifting-lug of the slag tar. The tipping leading arm on the slag jar will fall on the stanchion when certain titling angle is achieved and the slag jar will slowly rotate around the rotating shaft and pour the steel slag into the water-quench tank. At the same time, pressurized water in the granulator will be ejected and form a water screen to granulate the steel slag. The granulated slag is then stored in a slag storing area. The steel slag made from this invention has the characteristics that the granularity is small and the shape of grains is more regular. This invention does not pollute environment because the processed steel slag can be used as raw material and does not take a large space to store. Therefore, there is a need for a process and system for treating steel slag.